[postgis-users] Z coordinate value changes with st_transform
Richard Greenwood
richard.greenwood at gmail.com
Fri Mar 30 16:59:16 PDT 2012
Others more knowledgeable than myself should probably weight in on
this, but I don't believe the Z value from a horizontal datum
transform should be applied to an orthometric or ellipsoidal Z value.
Proj.4 returns a Z value which is a delta when transforming lat/long
(with no heights) between different datums when doing a 3 or 7
parameters datum transform (but not a grid shift tranform). 3 and 7
parameters transforms are done in XYZ geocentric space and the Z value
is just the "artifact" but does not represent a geographic change in
height.
The vertical datum support in Proj.4 is new and I don't think it is
what you are seeing here. If I'm correct in my assumptions then there
should be a ST_Transform() and a ST_Transform3D(). The Z value should
be disregarded in ST_Transform() and in ST_Transform3D() vertical
transformation should be explicitly requested from the Proj.4 lid.
Rich
On Fri, Mar 30, 2012 at 1:30 PM, Paul Ramsey <pramsey at opengeo.org> wrote:
> Turns out this is just because the definition of 23030 has been
> updated to include a to_wgs84 parameter. You get the same behavior in
> any version of proj. This is a feature, not a bug (our projection
> information is now better). If you want the old behaviour, change the
> proj4text of 23030 back to the 1.5 version.
>
> For a future release you might file a ticket to allow users to apply
> transforms x/y but to skip the z-coordinate.
>
> P.
>
> On Fri, Mar 30, 2012 at 10:27 AM, Paul Ramsey <pramsey at opengeo.org> wrote:
>> Yeah, bad news, 2.0 is showing this behavour but not 1.5.
>>
>> http://trac.osgeo.org/postgis/ticket/1742
>>
>> P.
>>
>> On Fri, Mar 30, 2012 at 9:59 AM, Stefano Iacovella
>> <stefano.iacovella at gmail.com> wrote:
>>> Hi Jose
>>>
>>> Im sure which version of PostGis are you using.
>>> On my box, Postgis 1.5.3 with Proj 4.8 it seems to work without any
>>> change to elevation:
>>>
>>> template_postgis=# select
>>> st_asewkt(st_transform(st_geomfromtext('POINT (700000 4500000 100)',
>>> 23030), 4326));
>>> WARNING: OGC WKT expected, EWKT provided - use GeomFromEWKT() for this
>>> st_asewkt
>>> ----------------------------------------------------------
>>> SRID=4326;POINT(-0.635451186670006 40.6259397158234 100)
>>> (1 row)
>>>
>>>
>>> template_postgis=# select postgis_full_version();
>>> postgis_full_version
>>>
>>> --------------------------------------------------------------------------------
>>> ------------------
>>> POSTGIS="1.5.3" GEOS="3.3.2-CAPI-1.7.2" PROJ="Rel. 4.8.0, 6 March 2012" LIBXML=
>>> "2.7.6" USE_STATS
>>> (1 row)
>>>
>>>
>>> Regards
>>>
>>> Stefano
>>>
>>> 2012/3/30 Jose Carlos Martinez <jomarlla at cgf.upv.es>:
>>>> I didnt get nay answers from now.
>>>> Someone can tell me why postgis is changing the z coordinate and how i can
>>>> change this default behavior?
>>>> Should I keep using proj4.7 instead of proj4.8?
>>>>
>>>>
>>>> Before:
>>>> pg20=# select st_astext(st_transform(st_geomfromtext('POINT (700000 4500000
>>>> 100)', 23030), 4326));
>>>> st_astext
>>>> --------------------------------------------------------------
>>>> POINT Z (-0.63662073736203 40.6248036513963 100)
>>>>
>>>>
>>>> Now:
>>>> pg20=# select st_astext(st_transform(st_geomfromtext('POINT (700000 4500000
>>>> 100)', 23030), 4326));
>>>> st_astext
>>>> --------------------------------------------------------------
>>>> POINT Z (-0.63662073736203 40.6248036513963 168.35361468256)
>>>>
>>>>
>>>>
>>>> On 27/03/2012 21:56, Jose Carlos Martinez Llario wrote:
>>>>>
>>>>> From proj4.8 changelog:
>>>>>
>>>>> "Preliminary support has been added for vertical datums using the
>>>>> +geoidgrids= directive. The geoidgrids are assumed to provide a
>>>>> transformation from geoid elevations to ellipsoidal elevations and they are
>>>>> discussed a bit in the VerticalDatums topic on the web site"
>>>>>
>>>>> I dont like the st_transform method can change my z coordinates by default
>>>>> though, dont know if it is because PostGIS is not ready still for proj4.8 or
>>>>> because it is the proj4.8 default behaviour.
>>>>>
>>>>> cheers,
>>>>> Jose
>>>>>
>>>>>
>>>>> On 27/03/2012 19:02, Jose Carlos Martinez wrote:
>>>>>>
>>>>>> Hi, dont know if this is the expected behavior, maybe its because I built
>>>>>> postgis with proj 4.8 or because something has changed in spatial_ref_sys.
>>>>>> Dont know at what time it happened (I missed it).
>>>>>>
>>>>>> Before:
>>>>>> pg20=# select st_astext(st_transform(st_geomfromtext('POINT (700000
>>>>>> 4500000 100)', 23030), 4326));
>>>>>> st_astext
>>>>>> --------------------------------------------------------------
>>>>>> POINT Z (-0.63662073736203 40.6248036513963 100)
>>>>>>
>>>>>>
>>>>>> Now:
>>>>>> pg20=# select st_astext(st_transform(st_geomfromtext('POINT (700000
>>>>>> 4500000 100)', 23030), 4326));
>>>>>> st_astext
>>>>>> --------------------------------------------------------------
>>>>>> POINT Z (-0.63662073736203 40.6248036513963 168.35361468256)
>>>>>>
>>>>>> Using ST_AsKml Its changing the Z coordinate too.
>>>>>>
>>>>>> Someone can confirm when this change happened in PostGIS?
>>>>>>
>>>>>> Regards,
>>>>>> Jose
>>>>>>
>>>>>>
>>>>>> _______________________________________________
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>>>>>
>>>>>
>>>>
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--
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com
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